Glosses and commentaries
Already in Late Antiquity, to sit in a class meant to read a text (lectio) under the guidance of a teacher. The notes of the teacher sometimes survive in the form of glosses (loose notes) or a full commentary, treating different aspects of the text.

Source: nog aanvragen
On the page, these notes and commentaries were organised around the text in particular fashions: the lay-out varies in relation to the extent of the commentary.
Organized lay-out
The page can be very well organized: neat spaces in the margin and in between the lines offer space to enter notes and glosses that are copied from one manuscript to the next. Marks link the note in the margin to the specific place in the main text to which it belongs.
About the manuscript
Leiden VLF 48
More about the manuscript VLF 48
Source
[[[48f30v.jpg]]]
[[[48f30v2.jpg]]]
Messy lay-out
Pages can also be completely overgrown with glosses and notes.Notes can end up on the wrong page, or be added on separate inserted slips of parchment. A page can be so densely glossed that it is difficult to see where one note ends, and the other begins, or to which phrase they belong
[[[12949f24v.jpg]]]
[[[12949f24v2.jpg]]] [[[12949f24v3.jpg]]] [[[12949f24v4.jpg]]]
Heavily of lightly glossed
On the page, a wealth of notes can be added, but also just a few. This can even vary per page: in the same manuscript, one page is completely filled, while the other is empty.Typically, the first (few) pages are heavily annotated, but this activity then dies out quickly.
[[[2788f49r.jpg]]] [[[2788f50v.jpg]]] [[[2788f52r.jpg]]]
What do the notes do?
In modern information theory, good information management ought to strive for four goals, the four s’s of information management: store, sort, select and summarize information (Ann M. Blair, Too much to know, p. 3) In medieval manuscripts, these four s’s are also happening in the margins. The notes are there, generally speaking, to help the reader, and storing, sorting, selecting and summarizing are important strategies.
In manuscripts, marginal notes are used to:
Call attention to certain things: 
Source: eigen foto
Source: eigen foto
Summarize and highlight the structure of the text Afbeelding ontbreekt
Explain and expand: 
Source: eigen foto
Refer to different authorities and compare: [[[48fdivers.jpg]]]
Visualize material in lists, diagrams and schemes: [[[12949fdivers.jpg]]]
Give the reader some guidance in how to interpret certain things: Afbeelding ontbreekt